This invention relates to a control rod and more particularly, to a control rod that can be used suitably for a boiling water reactor.
A control rod for use in a boiling water reactor has a crisscross sectional shape and four blades. Each blade is constituted by disposing a large number of neutron absorption rods in a sheath bent in a U-shape. Each sheath is fitted to a tie rod disposed at the axis of the control rod. The neutral absorption rods are aligned but are not fixed, inside the space defined by the tie rod, the sheath, an upper support member fitted to the upper portion of the tie rod and the upper surface of a velocity limiter fitted to the upper end portion of the tie rod.
The neutron absorption rod of the control rod, which is produced by packing B.sub.4 C powder into a cladding tube, involves the problem that its mechanical and nuclear life is short.
The problem described above can be solved to certain extents by the control rod disclosed in Japanese Patent Laid-Open No. 74697/1978 and U.S. Pat. No. 4,285,769 (FIG. 9D). In accordance with the prior art references, long life type neutron absorption rods having long nuclear and mechanical life are disposed at the upper end portion and blade end portions of the control rod where a neutron exposure is particularly large among the control rod. In other words, in the control rod of this kind, a long life type neutron absorber such as hafnium (Hf) or europium (Eu) is disposed at the upper end portion and blade end portion of the blade and a poison tube having boron carbide (B.sub.4 C) powder packed therein and fitted into a thinly elongated cladding tube made of stainless steel (SUS) is disposed at the rest of portions. However, this control rod is not yet free from the drawback that its service life is shorter than that of a control rod made solely of a long life type neutron absorber because the long life type neutron absorber is used only for part of the control rod.
To solve the problem described above, attempts have been made to replace the neutron absorber of the control rod by the long life type neutron absorber such as hafnium. One example of the control rods of this kind is illustrated in FIG. 4 of Japanese Patent Laid-Open No. 192,992/1984. In the control rod of this reference, all the neutron absorbers are made of hafnium, and solid hafnium is disposed at the upper portion of the rod while hollow hafnium is disposed at the lower portion. The solid hafnium region is positioned above a point which is 3/4 of the full length of the neutron absorber from its lower end.
In accordance with Japanese Patent Laid-Open No. 171,293/1982, on the other hand, a shock absorber or in other words, stainless pipes having a reduced thickness, are inserted horizontally between the upper end surface of the neutron absorber and an upper support member and between the lower end surface of the neutron absorber and the upper surface of a velocity limiter. The stainless pipes are disposed in order to mitigate the impact applied by the neutron absorber to the upper support member and to the velocity limiter at the time of ordinary driving or scram of the control rod.